1. Field of the Invention
The present invention relates to a printing apparatus and, more particularly, to a printing apparatus capable of performing overhead projector (OHP) processing for the purpose of enhancing light transmittance of ink in printing on an OHP sheet.
2. Description of Related Art
FIGS. 7(a) and 7(b) are views showing how a typical prior art ink jet printer operates. As depicted in FIG. 7(a), a printer head 17 comprises nozzles 63 filled with ink 62 and piezoelectric elements 64 furnished on the walls of the nozzles 63. On the surface of the printer head 17, the surface tension of the ink 62 prevents it from oozing out of the head 17.
A print command from a CPU 37 applies a voltage to appropriate piezoelectric elements 64. The voltage deforms the piezoelectric elements 64 in a convex fashion toward the corresponding nozzles 63, pressurizing the ink 62 within the nozzles 63, as illustrated in FIG. 7(b). When pressurized, a part of the ink 62 spurts out of the printer head 17 and adheres to a sheet 14.
When characters and figures are printed on an overhead projector (OHP) sheet 68, the ink 62 sticks protrusively on the surface thereof, as shown in FIG. 8(a). The protrusion of the ink 62 causes the light transmitting therethrough to scatter. The scattering of the transmitted light makes it difficult to obtain vivid colors of the characters and figures which are projected from the OHP sheet 68 onto a screen.
One method to solve the above problem is disclosed in U.S. Pat. No. 4,801,473. This method involves using a coating roller arrangement or the like to apply a coat of special coating material 70 over the printed surface of the OHP sheet 68. The coating smooths out the ink protrusions on the sheet surface after printing and eliminates the scattering of the light that passes through the OHP sheet 68, as shown in FIG. 8(b), whereby vivid colors of the print are obtained upon projection on the screen.
Another method proposed to address the same problem is to use a pressure roller assembly or the like to pressurize the printed OHP sheet 68. The roller flattens the protrusions of the ink 62, as illustrated in FIG. 8(c). The flattened ink eliminates the scattering of the transmitted light and ensures vivid colors of the print as it is projected on the screen.
Yet another method disclosed in U.S. Pat. No. 4,853,706 involves heating the printed OHP sheet 68 above a melting point of the ink 62. When heated, the ink 62 liquefies and spreads over the OHP sheet 68. When the ink 62 is cooled, the surface thereof becomes smooth and does not cause the transmitting light to scatter, as shown in FIG. 8(d). This also ensures vivid colors of the print as it is projected onto the screen.
One disadvantage of the proposed prior art methods is that it generally takes disproportionately longer for the printing apparatus operating using these methods to perform the so-called OHP processing than the time necessary to print the OHP sheet 68. The OHP processing, as described above, refers to the measures taken to render vivid the colors of the ink 62 as the print is projected from the OHP sheet 68 onto the screen. Another disadvantage of the prior art is that while the OHP sheet 68 printed in black obviously need not undergo OHP processing for higher transmittance, the prior art apparatus deals with both monochromatic (i.e., black) and multiple color sheets indiscriminately. The OHP processing of the prior art thus reduces a printing speed unavoidably and unnecessarily.